The present invention generally relates to wireless communication networks and in particular to a method of serving sector selection for packet data on a Forward Supplemental Channel (F-SCH).
An increasing trend in modern digital wireless communication networks, such as Code Division Multiple Access (CDMA) cellular networks, is the provision of data services, in addition to the traditional voice service. Data transmission may be circuit-switched, in which a logical channel is assigned to an individual mobile terminal for the duration of the data transfer, or packet-switched, in which data is packaged into logical addressable units (“packets”), which are transferred from point to point within the network until they reach their destination. Packet data may be transferred on a dedicated traffic channel to a single mobile terminal, or broadcast on a general channel, from which each mobile terminal may receive packets addressed to it.
The Forward Supplemental Channel (F-SCH) is a physical traffic channel that was introduced in the IS-2000 standard to allow the network to deliver high-speed data to mobile terminals, by allocating a large amount of radio resources, such as transmit power, Walsh space, and network resources to individual users. The resources include channel elements (CEs), BSC-BS backhaul bandwidth, and the like. In contrast to the Forward Fundamental Channel (F-FCH) operation, supporting F-SCH in soft handoff may not be efficient in term of maintaining system resource utilization and achieving high data throughput on various practical systems. The radio resources needed to support the F-SCH are proportional to the data rate on the F-SCH, and supporting soft handoff and/or a high data rate on the F-SCH can impose a significant toll on limited system resources. For example, a 2-way F-SCH soft handoff requires twice the physical resources (i.e., Walsh codes, CEs, and the maintenance of two data streams) as required by the F-SCH without soft handoff. Managing multiple data streams during soft handoff also adds complexity to the scheduling of data transmission. Moreover, complicated power control is needed to handle various active set change scenarios in order to maintain the desired quality on the F-SCH link.
Supporting F-SCH without soft handoff allows the system to maintain an efficient utilization of the limited system resources, since only one set of resources is allocated for the F-SCH at any time. It also simplifies the scheduling of data transmission as well as resource management design, and hence, reduces the overall system complexity and improves the stability. However, disabling soft handoff for F-SCH also presents technical challenges in maintaining the desired link quality for the F-SCH, especially when the mobile terminal is located in a soft handoff region or moves from one cell to another. Without soft handoff, as the mobile moves away from the F-SCH serving sector, the radio environment changes and the current serving sector cannot maintain a good quality F-SCH link to the mobile terminal, resulting in degraded data throughput for the mobile terminal. To compensate for the decreasing radio link quality, the F-SCH serving sector must either transmit with increasing power, which causes interference in the network, or the F-SCH data rate must be reduced, which reduces the throughput (or both). Eventually, either the mobile terminal or the network controllers will realize that there is very low throughput on the F-SCH link (such as by noting a high Frame Error Rate (FER)), and the F-SCH link is released. A new F-SCH link can then be brought up and a new serving sector can be selected. However, during the interim, the F-FCH must carry the high-speed data packets; thus, if the F-FCH is near its bandwidth, the throughput of packet data to the mobile terminal may suffer significantly.